Ortho esters diol protection

Orthogonal scheme, 359 2-Oxacydopentylidene ortho esters, to protect 1,2-diols, 137... 

Mono MOM derivatives of diols can be prepared from the ortho esters by diiso-butylaluminum hydride reduction (46-98% yield). In general, the most hindered alcohol is protected. ... 

A variety of cyclic ortho esters,including cyclic orthoformates, have been developed to protect czs-1,2-diols. Cyclic ortho esters are more readily cleaved by acidic hydrolysis (e.g., by a phosphate buffer, pH 4.5-7.5, or by 0.005-0.05 M HCl) than are acetonides. Careful hydrolysis or reduction can be used to prepare selectively monoprotected diol derivatives. 

The following ortho esters have been prepared to protect the diols of nucleosides. They are readily hydrolyzed with mild acjd to afford monoester derivatives, generally as a mixture of positional isomers. 

Thus, chalcone (26), available via aldol condensation between the appropriate benzaldehyde and acetophenone, was transformed into the 1,3-diarylpropene (27) via a two-step sequence involving ethyl chloroformate and NaBH4, followed by protection of the phenolic hydroxy group as the TBDMS ether. Asymmetric dihydroxylation of olefin (2 7) with AD-mix-a gave an intermediate diol, which was converted into ortho-ester (28) with triethyl orthoformate in the presence of catalytic pyridinium -toluenesulfonate (PPTS), followed by deprotection of the TBDMS ether with TBAF in THE. Treatment of ortho-ester (28) with triethyl orthoformate and PPTS gave an intermediate (27( ,35)- w j -flavan-3-ol formate ester. De-esterification with K2CO3 in THF/methanol and oxidation of the... 

The de novo Achmatowicz approach to the tris-rhamno portion of the anthrax tetrasaccharide began with the synthesis of disaccharide 115 from pyranone ent-44 and benzyl alcohol (Scheme 1.21). After glycosylation and postglycosylation transformations to install the rhamno-stereochemistry (ent-44 to 111), the 1,2-traus-diol of 111 was then protected with the Ley-spiroketal to provide monosaccharide 112 with a free C-2 hydroxyl group. After a similar three-step glycosylation (112 and 113) and postglycosylation sequence, 113 was converted into disaccharide 114, which in a one-pot ortho-ester protocol was protected to give disaccharide 115 with a free C-3 alcohol. 

Thus, the protective strategy in conjunction with living anionic polymerization successfully works to afford well-defined functional polystyrenes substituted with alcoholic and phenolic hydroxyl groups, diols, and triols. The silyl ether-, cyclic acetal-, and ortho ester-protected functionalities are effective for this purpose. This strategy may possibly be applied to other useful functional styrene derivatives and will be discussed in the next section. 

---